HALF MOON BAY -- Japanese radioisotopes aren't lurking in the sand at Miramar Beach, and playing there won't turn you into Spiderman.

Several months after a viral video suggested the beach contained radioactive material from the 2011 Fukushima Daiichi nuclear disaster, the California Department of Public Health has issued a final report confirming what its own preliminary analysis and independent studies had shown: radiation at the beach stems from naturally occurring elements in the sand, and it does not pose a threat to human health.

The video, which has been viewed by three-quarters of a million people, ricocheted around alternative news sites and brought to light a persistent divide: Despite firm assurances from scientists and various government agencies, some Californians remain concerned that radioactivity from Japan could contaminate their air, water and food.

"Nuclear radiation is something you can't smell, see and feel," said UC Berkeley nuclear engineering professor Kai Vetter, leader of the school's Rad Watch project, which has tested West Coast air, rain, milk and fish without finding any evidence that Fukushima-related contamination poses a health threat. "It tends to scare people."

Natural occurrence

The clip that drew attention to Miramar Beach -- "Fukushima radiation hits San Francisco?"-- was filmed in late December and depicted a man walking along the ocean just north of Half Moon Bay with a beeping Geiger counter. But the source of the radiation he detected is mundane.

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In a report released earlier this month, the Department of Public Health determined the culprits are small amounts of uranium, thorium and potassium contained in deposits of black sand. These elements, known as "primordial radionuclides," arrived on Earth during the planet's creation and are widely distributed throughout its crust.

Phillip Long, a geological project scientist with Lawrence Berkeley National Laboratory, said the phenomenon at Miramar Beach is nothing to be alarmed about.

"This is well within the range of normal differences in background radiation from one part of the country to another and one elevation to another," Long said of the radioactive sand. "I would never let it stop me from going to the beach."

The black sand likely contains monazite and zircon, heavy minerals found in granite and other igneous rocks that comprise much of the California coast, Long said. As the coastline erodes, those minerals wind up in the sand. Because they are denser than other sand particles, they tend to accumulate on beaches when lighter particles are sucked out to sea.

The streaks of dark sand are the only areas of the beach where radiation levels are elevated, and even in those spots the dose is relatively low. The highest dose recorded by public health investigators was about 0.1 millirem per hour. To put that in perspective, a transatlantic flight subjects passengers to about 2.5 millirems in radiation from outer space, which increases with elevation. A chest X-ray delivers about 10 millirems, according to the U.S. Nuclear Regulatory Commission, and a mammogram with two views yields 72 millirems.

American citizens receive an average radiation dose of about 620 millirems per year, half from natural background radiation and half from man-made sources. Most background exposure comes from radon, a gas produced by the decay of underground uranium and thorium. Medical procedures account for 96 percent of the dose from artificial sources.

Watching the ocean

Radioactive isotopes from the Fukushima meltdown spread to the western United States within days of the incident, according to Vetter, but in diluted form. In testing the air in Berkeley, milk produced in California, and salmon from Alaska, Vetter found that radioisotopes such as cesium 134 and 137 peaked in 2011 at levels far below public safety thresholds and have since declined to background concentrations.

Vetter and other scientists, such as Ken Buesseler of the Massachusetts-based Woods Hole Oceanographic Institution, are now watching for ocean-borne cesium, which is expected to arrive this spring on the West Coast in low concentrations -- several hundred times below federal standards for drinking water. Vetter is participating in a project to monitor cesium and other isotopes in kelp from Baja California to Alaska, while Buesseler is testing the ocean along the California coast.

As for Miramar, Vetter said he wouldn't hesitate to let his children play in the sand, which is what Anne Crossman was doing Friday afternoon. She stayed away after the video first came out, but once the Fukushima link was debunked, she decided any risks were outweighed by the physical and emotional benefits to her four children, ages 1 to 8, of playing by the ocean.

"I feel like everywhere I turn there's a scare about something else that's in the water or our food, especially when it comes to young children," said Crossman, 36. "I need to be attentive, and I am. But if I listened to everything, I'd never leave the house."